Heart Rate Variation During Treadmill Walk

[Tevye]

Is there a physiological explanation for the variation in my heart rate (see image, below) during my treadmill session?  I do this daily: one hour walk on my treadmill @2.3 mph, 1st three minutes at zero degrees elevation, next three minutes at 3 deg. elevation, and final 54 minutes at 4 deg. elevation.  Same routine every day.  And same variation of my heart rate: about 110 bpm for approximately the first 60% of the session and then jumping up to about 130 bpm for the remaining 40% of the session.  Same result every day.  Why the jump up in heart rate for the latter part of the session?

Appreciate any insights on this!

Tevye

[WavyDavey]

Hi @Tevye

I think that's a pretty typical graph. An increase from 100 to 130bpm isn't huge. I believe there are (at least) three reasons that sustained exercise gradually increases heart rate:

1. Increased blood flow needed to cool the body as temperature builds during sustained exercise.

2. Increased blood flow needed to deliver oxygen rich-blood to muscles.

3. Increased blood flow to transport waste such as lactic acid away from muscles.

Looks like you're getting good workouts. Keep on walking!

[Tevye]

Thanks Dave!

[Heybales]

I'll add:

#4 - Cardiovascular drift - the effect of doing the exact same workload (pace/incline) and outside any other reasons listed above (hot, dehydrated) the HR slowly and steadily goes up.

An hour is about right to start seeing that effect. Now at the start you do increase the workload with incline.

There have been studies on people kept at same temp, same hydration level, that experience this.

Throw all those together and you likely have some very expected reasons why it would be so consistent.

While this effect is slow and generally steady, it's probably gone up right as one of the other reasons hit.

Probably could do a session with less water than normal, or hotter than normal - and see just what effect that has on it. Actually, instead of hotter, if at gym take that group fan and point it right at you explaining "doing science, thank you for your support". 😉

Ever varied your turnover at same pace?

Fascinating how even that shows up more or less efficient movement and can change HR.

Usually running is easier to see that effect, not sure I ever tried it while walking.

[Tevye]

Thanks Heybales!  The treadmill is in my basement and I have a good floor fan.  I'll give it a try!

Tevye

[emili]

to add my two cents.. it is often the case in steady state cardio. the longer you do the same cardio the "harder" your heart will work. It is the same thing that happens to runners. you go at a steady pace and at the start you are feeling good and there is normal effort. The longer you go, the harder you have to work to keep the pace and the heart rate goes up. I find this for myself during longer runs. Everything is working as it should be. You will also notice that for a little bit after your cardio, your HR is slightly higher doing normal things like walking upstairs to get changed or anything that isn't sedentary. 

[Heybales]

I need to correct myself here, as my info didn't sound totally correct on rereading, and it's been awhile since I had looked into this.

#4 I mentioned is mainly an effect of #1 - increased body core temp - need some cooling effect.

A few other reasons not related to #1-3, but #1 is majority of reason.

The HR increases, but since more oxygen isn't needed since the workload didn't actually go up, the stroke volume per heart beat goes down to compensate.

This can be noticed by the breathing rate not going up much at all.

Doesn't lower stroke volume to pre-hot levels, as the capillaries near skin surface for cooling require some extra volume. And some blood volume may have been lost from sweating related to hot core.

But the negative to this effect which can be major gotcha is the HRM is seeing increased HR.

So now this already perhaps sketchy method of calorie burn thinks you are burning more - when the workload is exactly the same.

So inflated calorie burn.

Which means if properly eating more when you do more - may be eating into that deficit which will slow weight loss. If already at a reasonable rate of loss for amount to lose - that could be a problem.

You could look at the calorie burn for each half of your workout with equal pace and see how bad that effect is.

Or pick a 20 min period before it increases, see the rate of burn, and apply that to the whole thing.

Then correct each workout when finished.

And for others reading, the other issue is training - if hitting a certain workload based on HR (trying to stay in a HR zone, or equipment that does it for you automatically) - then workload (pace/speed) would be lowered to keep the HR lower, but then you are missing the benefit of the workout since it's not actually pushing the body as hard as it could go for good workout.

[Tevye]

Thanks Elena!  It's good to know that the HR increase is an expected effect during steady-state exercise.

Tevye

[Tevye]

Thanks for the additional info, Heybales!  I looked back over several days of my exercise plots in Fitbit and the calorie burn-rate graph does increase slightly when the HR graph moves up into the peak zone in the latter part of my sessions.

Tevye